(a) Field of the Invention
This invention relates to a general method of seismic elastic inversion and more particularly, but not by way of limitation, to a specific inversion method designed to combine compressional interval velocity from seismic data and pseudo shear interval velocity data for predicting fluid type and porosity.
(b) Discussion of Prior Art
In the exploration of oil and gas using seismic data, forward and inverse methods are used. A forward method is where conditions and experiment are known and the results are measured and repeatable. An inverse method is opposite of a forward method and has multiple solutions.
When using an inverse method, and more specifically the inversion method discussed herein, the end results and experiment are known and the original geological conditions are unknown and are to be solved. It is true that multiple solutions for different conditions can give rise to the same measurements. Also, it is possible to narrow the choice of the different measurements.
Most geological problems can in fact be analyzed using inverse procedures. Understanding the relationship of the inverse procedures is both simple and profound in seismic data study. Therefore, using an inverse method, as described herein, can be a fundamental reason for helping reduce risk and uncertainty in the study of geology and geophysics related to oil and gas exploration.
In the process of interpretation of seismic inversion data, it is essential to first recognize that inverse methods can have multiple solutions, as mentioned above, and then understand a range of possible variables in a forward method to evaluate an answer. In the subject inversion method, (which is a numerical method), seismic response is used to predict rock properties such as velocity, density, compressibility, porosity and water saturation. Heretofore, there have been many other and different inversion methods that also claim to be able to do this. The subject method differs from those other methods in that it is calibrated and as a result more accurate. It can be used to distinguish between non-commercial “Fizzgas” and commercial gas accumulations prior to drilling.
The subject inversion method is classified as a general elastic inversion method rather than an acoustic method, since it uses shear information. Also and unlike shape attribute methods, the inversion method can be calibrated, scaled to well control and the results are non-arbitrary.